1. Field of the Invention
The present invention relates to a system for controlling the operation of a turbo-type fluid machine. More particularly, the present invention relates to a turbo type fluid machine operation control system which enables a turbo machine, e.g., a pump, a blower, etc., to exhibit favorable performance characteristics in conformity to each particular purpose of use.
2. Description of the Related Art
To control the output of a turbo-type fluid machine, e.g., a pump, a blower, etc., a technique whereby the rotational speed of a driving machine for the turbo machine is varied has been widely employed. According to a conventional method of controlling the output of a turbo machine by the rotational speed control, the rotational speed is varied so that the head or pressure produced by the turbo machine or the capacity (i.e., discharge quantity) thereof complies with the demand therefor, thereby controlling the characteristics of the fluid machine. That is, in automatic control, the rotational speed is feedback-controlled on the basis of the result of a comparison between the value of the detected quantity, e.g., the discharge pressure, capacity, etc., on the demand side and the corresponding set value.
For example, to change the characteristics of a turbo type fluid machine that is driven by an internal combustion engine or other driving machine, such a method has been put to practical use whereby the fuel feed is controlled by a speed governor to thereby control the rotational speed of the driving machine. To set a rotational speed, a signal is manually or automatically given to the speed governor according to the required discharge pressure or capacity of the fluid machine. However, no method has been known whereby a pump and/or a driving machine therefor is allowed to exhibit its capacity to the maximum in accordance with the operating conditions, as is demanded for a drainage pump or the like, except for constant-speed shaft power control that is carried out by employing adjustable vanes incorporated in the pump. It should be noted that when adjustable vanes are employed, a complicated mechanism is needed to adjust the angle of the vanes. Therefore, the constant-speed shaft power control is disadvantageous in general-purpose applications.
For a pump that is driven by an induction motor, a method has heretofore been put to practical use wherein, in order to change pump characteristics in accordance with the operating conditions and load conditions, the frequency of the power supply for the motor is changed by the use of a static invertor, thereby controlling the rotational speed. In this regard, for example, Japanese Patent Application Public Disclosure (KOKAI) Nos. 57-52396 and 59-44997 disclose a method wherein the invertor is controlled so that the operating efficiency of the motor is maximized. Japanese Patent Application Public Disclosure (KOKAI) No. 59-25099 discloses a control method wherein the motor output is maintained at the rated value by an invertor. Japanese Patent Application Post-Examination Publication No. 57-113992 discloses a method wherein a rotational speed that gives a target pressure is arithmetically obtained to control the speed of a pump.
However, none of these conventional techniques intend to positively improve the performance characteristics of a turbo-type fluid machine so that the characteristics become favorable for actual use while allowing the combination unit of the fluid machine and a driving machine therefor to exhibit its capacity to the full.
As is generally known, when high specific speed turbo-type fluid machines are operated at constant speed, the required power decreases as the operating head or pressure decreases and the capacity increases. Particularly, when the capacity is above the rated value, the required power markedly decreases as the operating head or pressure decreases. That is, in this operating region, the allowance for the capability of the driving machine increases; therefore, the capacity of the driving machine cannot fully be exhibited. Further, in these fluid machines, the required power increases when the capacity is below the rated value. Therefore, in a case where the machine is expected to operate in this region, sufficient allowance must be given to the rated output of the driving machine in order to avoid overloading of the driving machine.
In the case of low specific speed turbo-type fluid machines, on the other hand, when a constant-speed operation is performed, the capacity decreases as the operating head or pressure increases, and the required power also decreases. That is, in this operating region, the allowance for the capability of the driving machine increases; therefore, the capacity of the driving machine cannot fully be exhibited. Further, in these fluid machines, the required power increases when the capacity is above the rated value. Therefore, it is necessary to give allowance to the rated output of the driving machine. Further, in the case of a low specific speed fluid machine in which the head (pressure)-capacity curve is flat, the capacity varies to a considerable extent with a change in the operating head or pressure, so that a problem is likely to arise in receiving systems or processes at the downstream side. Accordingly, it is necessary to limit the capacity by discharge valve throttling control or other similar controls.
Further, the pressure-capacity curve assumes a convex shape with a maximum value in a small capacity region as is observed in a centrifugal blower or the like. Therefore, the operational range is limited to the capacity region above the capacity value at which the pressure reaches a maximum in order to avoid surging.